1. Field of the Invention
This invention relates to the manufacture of panels with regularly shaped (usually hexagonal) cells sometimes known as honeycomb panels.
2. Description of Related Art
The manufacture of honeycomb panels from sheet material can be carried out employing various sheet materials. Historically the sheet material was pre-corrugated and the corrugated sheets were adhered along the land areas. This, however, required good registration of the cells and tends now only to be used for high modulus materials such as metal sheets.
For paper or similar fibrous sheet materials a general method comprises printing or applying parallel lines known as node lines of adhesives. The printed sheet material is then cut into individual sheets which are stacked so that the node lines on adjacent sheets are displaced from each other by one half the pitch (distance between) of the lines. The resulting stack can contain several hundred to several thousand sheets depending on the size of the final honeycomb block required. Depending on the nature of the adhesive means, these adhesives are then set so as to join the sheets and the resulting block is expanded by pressure on the sides of the block at right angles to the direction of the node lines which force expansion, i.e., separation of the sheets at each cell point to create a cellular structure. The resulting block is then cut into panels by cutting at right angles to the node lines to form a core for a composite panel. Surface sheets may then be applied on one or both sides of the core across the surface of the open cells to form a composite panel.
These cores have found wide usage in many industries for such applications as packaging, furniture, display panels and in general any application for which there is a requirement for a stiff, yet lightweight element.
Originally, these cores were made from Kraft-type papers in which the paper walls of the cells were bonded together with simple adhesives with or without a subsequent coating of a polymeric resin over the entire surface of the core material.
However, paper honeycomb materials suffer from a major disadvantage in that they have low strength, particularly in compression. When resins are incorporated into such paper core materials, the effect is to improve considerably the compression strength, but the resulting core material is then very brittle and has poor resilience.
A further major disadvantage of cellulose based paper honeycombs is their poor fire resistance. Techniques for imparting fire-resistance have included impregnating a core made from relatively porous paper, e.g., Kraft paper with a water-based phenolic resin and then coating with a polymer (for example in latex form) which contains a dispersed particulate flame retardant. This is described in U.K. Patent GB-A-1 444 346. The first resin coating renders the core paper more moisture resistant but as it is Kraft paper the core is very brittle. Also the cores tend to be very heavy because formed from Kraft paper have mechanical properties. Paper has been tried which contains flame retardant but this is more difficult to print with node adhesive and is more stiffer making expansion more difficult. Japanese Patent JP 06272190, A2 940 927 (Nettetsu Mining Co.) describes a sheet of 65-80% fireproofing powder, 3477 flame-retardant resin powder and 15-25 cellulose fibers but this is essentially an inorganic sheet not a cellulosic fiber paper. In another patent (JP 08103979 A2 960 423) of the same company, a core from this sheet, formed into a honeycomb, is impregnated with fire retardants such as guanidine phosphate.
In recognition of these deficiencies, the composites industry developed honeycomb core materials based on other paper-like materials, and especially aramid paper, i.e., one made from fibers of highly aromatic polyamide resins. The best known example is Nomex.RTM. poly (m-phenylene isophthalamide) (product of DuPont). These aramid papers comprise one or more fibers, e.g., Nomex.RTM. or Kevlar.RTM. in the form of fibers and so-called fibrids which are formed into an impenetrable sheet. The development of such cores meant that honeycomb structures could now be used in more demanding applications such as flooring for aircraft and ships, components for skis and snowboards and other applications where a light, stiff, high toughness structural material is required. However, aramid papers are much higher in cost than paper, and in many applications this cost prohibits the use of aramid cores.
Additional factors are the weight and poor properties of the conventional papers (Kraft papers) employed. It would be desirable to improve the weight (density) of the core and the strength of the cellular structure.
In the manufacture of an aramid paper honeycomb, the stack or block of sheets after placing in a heated press and curing the node line adhesive is removed from the press and expanded to form hexagonal cells. A block of aramid paper prior to expansion can be sprayed with water to facilitate expansion but this cannot be applied to cellulose papers, such as Kraft paper, as wet paper cannot be expanded as it collapses. The expanded block of aramid paper is set in this shape by heating in the expanded state to a temperature exceeding the glass transition temperature (Tg) of the aramid resin and then cooling. It is not possible to "heat set" cellulose-based papers in this way because cellulose degrades thermally at temperatures well below its Tg. However, it would still be desirable to be able to set the shape of a paper honeycomb, because otherwise once expanded the honeycomb will collapse. Also, it is desirable to be able to remove expanded paper honeycomb from its expansion frame prior to resin coating. Although possible, it is generally not advantageous to resin coat the expanded honeycomb while it is attached to the expansion frame. This is because of the problem of removing cured resin from the frame after the curing process has been carried out.
Heat-setting on the expansion frame has been affected with Kraft paper. This may be possible because of the inherent moisture content of Kraft paper. In many cases, however, there is no necessity to stabilize the Kraft paper core as it is continuously expanded and fed into its intended final position.